Wireless power transfer

ABSTRACT

A power receiver receives a wireless power transfer from a power transfer signal generated by a wireless power transmitter during a power transfer phase. The power transfer signal employing a repeating time frame during the power transfer phase where the frame comprises at least a power transfer time interval and a foreign object detection time interval. The power receiver comprises a synchronizer ( 311 ) which synchronizes a local time reference to the repeating time frame and a load controller ( 309 ) which disconnects a load ( 303 ) during at least part of the foreign object time detection time intervals during at least part of the power transfer phase. The timing of the disconnecting is dependent on the local time reference. A mode controller ( 313 ) switches between a first operational mode and a second operational mode for the power transfer time intervals in response to a reliability measure for the synchronization.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application a continuation of U.S. application Ser. No. 17/054,862filed on Nov. 12, 2020, which claims the benefit of InternationalApplication No. PCT/EP2019/062540 filed May 15, 2019, which claims thebenefit of EP Application No. 18172577.1 filed May 16, 2018. Theseapplications are hereby incorporated by reference herein.

FIELD OF THE INVENTION

The invention relates to operation of a wireless power transfer systemand in particular, but not exclusively, to foreign object detection in awireless power transfer system.

BACKGROUND OF THE INVENTION

Most present-day electrical products require a dedicated electricalcontact in order to be powered from an external power supply. However,this tends to be impractical and requires the user to physically insertconnectors or otherwise establish a physical electrical contact.Typically, power requirements also differ significantly, and currentlymost devices are provided with their own dedicated power supplyresulting in a typical user having a large number of different powersupplies with each power supply being dedicated to a specific device.Although, the use of internal batteries may avoid the need for a wiredconnection to a power supply during use, this only provides a partialsolution as the batteries will need recharging (or replacing). The useof batteries may also add substantially to the weight and potentiallycost and size of the devices.

In order to provide a significantly improved user experience, it hasbeen proposed to use a wireless power supply wherein power isinductively transferred from a transmitter inductor in a powertransmitter device to a receiver coil in the individual devices.

Power transmission via magnetic induction is a well-known concept,mostly applied in transformers having a tight coupling between a primarytransmitter inductor/coil and a secondary receiver coil. By separatingthe primary transmitter coil and the secondary receiver coil between twodevices, wireless power transfer between these becomes possible based onthe principle of a loosely coupled transformer.

Such an arrangement allows a wireless power transfer to the devicewithout requiring any wires or physical electrical connections to bemade. Indeed, it may simply allow a device to be placed adjacent to, oron top of, the transmitter coil in order to be recharged or poweredexternally. For example, power transmitter devices may be arranged witha horizontal surface on which a device can simply be placed in order tobe powered.

Furthermore, such wireless power transfer arrangements mayadvantageously be designed such that the power transmitter device can beused with a range of power receiver devices. In particular, a wirelesspower transfer approach, known as the Qi Specifications, has beendefined and is currently being developed further. This approach allowspower transmitter devices that meet the Qi Specifications to be usedwith power receiver devices that also meet the Qi Specifications withoutthese having to be from the same manufacturer or having to be dedicatedto each other. The Qi standard further includes some functionality forallowing the operation to be adapted to the specific power receiverdevice (e.g. dependent on the specific power drain).

The Qi Specification is developed by the Wireless Power Consortium andmore information can e.g. be found on their website:http://www.wirelesspowerconsortium.com/index.html, where in particularthe defined Specification documents can be found.

In power transfer systems, such as Qi, the electromagnetic fieldgenerated to transfer the required levels of power to the power receiveris often very substantial. The presence of such a strong field may inmany situations have an impact on the surroundings.

For example, a potential problem with wireless power transfer is thatpower may unintentionally be transferred to e.g. metallic objects thathappen to be in the vicinity of the power transmitter. For example, if aforeign object, such as e.g. a coin, key, ring etc., is placed upon thepower transmitter platform arranged to receive a power receiver, themagnetic flux generated by the transmitter coil will introduce eddycurrents in the metal objects which will cause the objects to heat up.The heat increase may be very significant and may be highlydisadvantageous.

In order to reduce the risk of such scenarios arising, it has beenproposed to introduce foreign object detection where the powertransmitter can detect the presence of a foreign object and reduce thetransmit power and/or generate a user alert when a positive detectionoccurs. For example, the Qi system includes functionality for detectinga foreign object, and for reducing power if a foreign object isdetected. Specifically, Qi specification version 1.2.1, section 11describes various methods of detecting a foreign object.

One method to detect such foreign objects is disclosed inWO2015018868A1. Another example is provided in WO 2012127335 whichdiscloses an approach based on determining unknown power losses. In theapproach, both the power receiver and the power transmitter measuretheir power, and the receiver communicates its measured received powerto the power transmitter. When the power transmitter detects asignificant difference between the power sent by the transmitter and thepower received by the receiver, an unwanted foreign object maypotentially be present, and the power transfer may be reduced or abortedfor safety reasons. This power loss method requires synchronizedaccurate power measurements performed by the power transmitter and thepower receiver.

For example, in the Qi power transfer standard, the power receiverestimates its received power e.g. by measuring the rectified voltage andcurrent, multiplying them and adding an estimate of the internal powerlosses in the power receiver (e.g. losses of the rectifier, the receivercoil, metal parts being part of the receiver etc.). The power receiverreports the determined received power to the power transmitter with aminimum rate of e.g. every four seconds.

The power transmitter estimates its transmitted power, e.g. by measuringthe DC input voltage and current of the inverter, multiplying them andcorrecting the result by subtracting an estimation of the internal powerlosses in the transmitter, such as e.g. the estimated power loss in theinverter, the primary coil, and metal parts that are part of the powertransmitter.

The power transmitter can estimate the power loss by subtracting thereported received power from the transmitted power. If the differenceexceeds a threshold, the transmitter will assume that too much power isdissipated in a foreign object, and it can then proceed to terminate thepower transfer (or to adjust the operating parameters accordingly, e.g.restrict the power transfer to be below a given level).

Alternatively, it has been proposed to measure the quality or Q-factorof the resonant circuit formed by the primary and secondary coilstogether with the corresponding capacitances and resistances. Areduction in the measured Q-factor may be indicative of a foreign objectbeing present.

In practice, it tends to be difficult to achieve sufficient detectionaccuracy using the methods described in the Qi specification. Thisdifficulty is exacerbated by a number of uncertainties about thespecific current operating conditions.

For example, a particular problem is the potential presence of friendlymetals (i.e. metal parts of the device that embodies the power receiveror the power transmitter) as the magnetic and electrical properties ofthese may be unknown (and vary between different devices) and thereforemay be difficult to compensate for.

Further, undesirable heating may result from even relatively smallamounts of power being dissipated in a metallic foreign object.Therefore, it is necessary to detect even a small power discrepancybetween the transmitted and received power, and this may be particularlydifficult when the power levels of the power transfer increase.

The Q factor degradation approach may in many scenarios have a bettersensitivity for detecting the presence of metal objects. However, it maystill not provide sufficient accuracy and e.g. may also suffer from theinfluence of friendly metal.

The performance of the foreign object detection is subject to thespecific operating conditions that are present when the test is actuallyperformed. For example, if, as described in the Qi specification, ameasurement for foreign object detection is performed in the SelectionPhase of a power transfer initialization process, the signal that thepower transmitter provides for the measurement must be small enough toprevent that it wakes up the power receiver. However, for such a smallsignal, the signal/noise ratio is typically poor, resulting in reducedaccuracy of the measurement.

The requirement for a small measurement signal may result in otherdisadvantageous effects. A power receiver exposed to a small measurementsignal may exhibit a leakage current that depends on the level of themeasurement signal, the coupling between the primary and secondary coil,and the charging state of a capacitor at the output of the rectifier.This leakage current can therefore be different depending on the actualconditions. Since leakage current influences the reflected impedance atthe power transmitter coil, the measurement of the quality factor willalso depend on the specific current conditions.

Another issue is that foreign object detection is typically a verysensitive test where it is desired that relatively small changes causedby the presence of a foreign object is detected in an environment withpossibly a large variation of the operating conditions and scenarios forwhich the test is being performed.

Accordingly, current algorithms tend to be suboptimal and may in somescenarios and examples provide less than optimum performance. Inparticular, they may result in the presence of foreign objects not beingdetected, or in false detections of foreign objects when none arepresent.

The difficulties of accurate foreign object detection are particularlydifficult in scenarios wherein the power level of the power transfersignal is high and/or when it varies. Thus, foreign object detection isparticularly difficult during the power transfer phase, and especiallyfor power receivers that represent a large and varying load. Further,there tends to be conflicting requirements for the foreign objectdetection and the power transfer and indeed the power transfer oftentends to interfere with the foreign object detection. However, modifyingthe power transfer operation to improve the foreign object detectiontends to have a detrimental impact on the power transfer.

Other operations of the power transfer system may furthermore besensitive to such effects. For example, in many situations,communication between the power transmitter and power receiver may benegatively affected by large loads, and in particular by large loadvariations.

In many systems, communication from the power receiver to the powertransmitter may use load modulation where a load of the power transfersignal is varied in dependence on the data to be transmitted. However,such load modulation may be difficult to detect if the power transferloading of the power transfer signal varies at the same time. Similarly,communication from the power transmitter to the power receiver may beachieved by modulating the power transfer signal (e.g. amplitude orfrequency modulation) but interference to such modulation may be causedby variations in the parameters of the power transfer signal due to avarying load.

Indeed, even if a completely separate carrier is used for communication,such as a NFC communication link, a very large and varyingelectromagnetic field caused by the power transfer signal may causesubstantial interference despite being in a very different frequencyband.

Thus, the presence of the power transfer signal, and the loadingthereof, may have detrimental impact on other operations, such asforeign object detection and communication operations.

Hence, an improved operation for a power transfer system would beadvantageous, in particular, an approach allowing increased flexibility,reduced cost, reduced complexity, improved foreign object detection,improved communication, improved support for different loads, improvedadaptability, backwards compatibility, reduced impact on the powertransfer operation, improved power transfer operation, and/or improvedperformance would be advantageous.

SUMMARY OF THE INVENTION

Accordingly, the Invention seeks to preferably mitigate, alleviate oreliminate one or more of the above mentioned disadvantages singly or inany combination.

According to an aspect of the invention there is provided power receiverfor receiving wireless power transfer from a power transfer signal froma wireless power transmitter during a power transfer phase, the powertransfer signal during the power transfer phase employing a repeatingtime frame comprising at least a power transfer time interval and aforeign object detection time interval; the power receiver comprising: asynchronizer for synchronizing a local time reference to the repeatingtime frame; a load controller for disconnecting a load during at leastpart of the foreign object time detection time intervals during at leastpart of the power transfer phase, a timing of the disconnecting beingdependent on the local time reference; a mode controller for switchingbetween a first operational mode and a second operational mode for thepower transfer time intervals in response to a reliability measure forthe synchronization, wherein different power transfer parameters areemployed in the first operational mode and in the second operationalmode.

The invention may provide improved performance in many embodiments, andmay provide an overall improved power transfer operation in many systemsand embodiments. For example, in many embodiments, foreign objectdetection may be achieved by performing such detections during timeintervals created to provide particularly advantageous conditions.

The approach may in many embodiments reduce complexity, and may in manysystems provide a high degree of backwards compatibility. Specifically,the approach may be particularly suitable for improving foreign objectdetection in Qi wireless power transfer systems e.g. operating inaccordance with version 1.2 or earlier of the Qi Specifications.

The invention may in many embodiments provide a more reliable and/orsafe operation. In particular, it may mitigate and reduce the risk ofover-voltage conditions occurring when disconnecting the load. Forexample, the power level of the power transfer signal during the powertransfer time intervals may be reduced when the reliability measureindicates that synchronization is not reliable thereby reducing the riskof unacceptable over-voltage being induced if disconnection occursduring a power transfer time interval.

In many embodiments, a duration of the foreign object detection timeinterval is no more than 5%, 10%, or 20% of the duration of the timeframe. In many embodiments, the duration of the foreign object detectiontime interval(s) is no less than 70%, 80%, or 90% of the time frame.

During the foreign object detection time interval, the power level ofthe power transfer signal may be reduced corresponding to the level ofpower being transferred from the power transmitter to the power receiverbeing reduced. During the foreign object detection time interval, apower level of power transferred from the power transmitter to the powerreceiver may be reduced relative to a power level of power transferredfrom the power transmitter to the power receiver during the powertransfer time interval. The power level, and references to power andpower level, may specifically be considered to relate to the real power(I·U·Cos ϕ).

The first and second operational modes may employ different powertransfer parameters by at least one of: applying a different limit for asignal level of the power transfer signal in the first and secondoperational modes; employing a different load of the power transfersignal in the first and second operational modes; and applying adifferent limit to rate of changes for power levels of the powertransfer signal in the first and second operational modes. Thesynchronizer may be arranged to generate the reliability measure for thesynchronization.

In accordance with an optional feature of the invention, thesynchronizer is arranged to perform a synchronization of the local timereference to the repeating time frame when entering the power transferphase, the mode controller is arranged to control the power receiver tooperate in the first operational mode when entering the power transferphase, and to switch the power receiver to the second operational modein response to a detection that the reliability measure for thesynchronization exceeds a threshold.

This may provide improved operation in many embodiments includingtypically a more reliable and/or safe power transfer operation and/orimproved foreign object detection. The approach may in particular allowimproved power transfer initialation for scenerios wherein the powerreceiver seeks to extract a large amount of power during the powertransfer phase.

In accordance with an optional feature of the invention, the powerreceiver further comprises a signal level controller for transmittingsignal level requests for the power transfer signal to the powertransmitter; the signal level controller being arranged to control thesignal level of the power transfer signal during the power transferintervals to differ from a signal level of the power transfer signalduring the foreign object detection intervals when in the firstoperational mode; and wherein the synchronizer is arranged tosynchronize in response to signal variations between the power transfertime intervals and the foreign object time intervals.

This may provide improved performance in many embodiments and mayprovide a reliable initial timing adaptation and synchronization.

In accordance with an optional feature of the invention, thesynchronizer is arranged to determine the reliability measure for thesynchronization in response to a duration of operation in the first modeof operation.

This may provide improved performance in many embodiments.

In accordance with an optional feature of the invention, the powerreceiver further comprises an initiator which is arranged to determine aset of parameters for the foreign object time intervals by communicatingwith the power transmitter prior to entering the power transfer phase,the set of parameters comprising at least one of: a. a duration of theforeign object time intervals; b. an interval between foreign objecttime intervals; and c. a signal level for the foreign object detectionintervals.

This may provide for a more reliable operation and may in particularallow adaptation of the operation to the specific conditions.

In accordance with an optional feature of the invention, thesynchronizer is arranged to perform the synchronization based on the setof parameters.

This may provide improved performance and may allow an advantageousadaptation of the operating conditions with parameters being determinedby negotiation and subsequent adjustment by e.g. automaticsynchronization.

In accordance with an optional feature of the invention, thesynchronizer is arranged to determine the reliability measure inresponse to a comparison of a timing parameter for the foreign objectdetection time intervals determined from the local time reference andcorresponding timing parameter of the set of parameters.

This may provide a particularly advantageous determination of thereliability measure.

In accordance with an optional feature of the invention, the powerreceiver is arranged to control the power transmitter to limit a signallevel of the power transfer signal to a level which is lower when in thefirst mode of operation than when in the second mode of operation.

This may provide improved and/or more reliable operation in manyembodiments and may in particular mitigate and/or reduce the risk ofover-voltage conditions.

In accordance with an optional feature of the invention, the loadcontroller is arranged to disconnect a load from the power transfersignal during power transfer time intervals when in the first mode ofoperation but not when in the second mode of operation.

This may provide improved operation.

In accordance with an optional feature of the invention, the powerreceiver further comprises a power level controller for transmittingpower level requests for the power transfer signal to the powertransmitter; the power level controller being arranged to limit a rateof change for power levels when in the first mode of operation to alower level than when in the second mode of operation

In accordance with an optional feature of the invention, thesynchronizer is arranged to determine the reliability measure for thesynchronization in response to a comparison of signal levels for thepower transfer signal during power transfer time intervals and foreignobject time intervals.

This may provide a particularly advantageous determination of thereliability measure.

In accordance with an optional feature of the invention, the modecontroller is arranged to switch the power receiver from the secondoperational mode to the first operational mode in response to adetection that the reliability measure for the synchronization is belowa threshold.

In accordance with an optional feature of the invention, the loadcontroller is arranged to reconnect the load during the foreign objecttime detection time intervals during at least part of the power transferphase, a timing of the reconnecting being dependent on the local timereference.

In accordance with an optional feature of the invention, the powerreceiver further comprises a current restrictor for restricting acurrent to the load when reconnecting the load.

According to an aspect of the invention there is provided a method ofoperation for a power receiver receiving wireless power transfer from apower transfer signal from a wireless power transmitter during a powertransfer phase, the power transfer signal during the power transferphase employing a repeating time frame comprising at least a powertransfer time interval and a foreign object detection time interval; themethod comprising: synchronizing a local time reference to the repeatingtime frame; disconnecting a load during at least part of the foreignobject time detection time intervals during at least part of the powertransfer phase, a timing of the disconnecting being dependent on thelocal time reference; and switching between a first operational mode anda second operational mode for the power transfer time intervals inresponse to a reliability measure for the synchronization, whereindifferent power transfer parameters are employed in the firstoperational mode and in the second operational mode.

These and other aspects, features and advantages of the invention willbe apparent from and elucidated with reference to the embodiment(s)described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only,with reference to the drawings, in which

FIG. 1 illustrates an example of elements of a power transfer system inaccordance with some embodiments of the invention;

FIG. 2 illustrates an example of elements of a power transmitter inaccordance with some embodiments of the invention;

FIG. 3 illustrates an example of elements of a power receiver inaccordance with some embodiments of the invention;

FIG. 4 illustrates an example of elements of an output stage of a powertransmitter;

FIG. 5 illustrates an example of elements of a power receiver inaccordance with some embodiments of the invention;

FIG. 6 illustrates an example of a time frame for a wireless powertransfer system of FIG. 1; and

FIG. 7 illustrates an example of a time frame for a wireless powertransfer system in accordance with some embodiments of the invention;

FIG. 8 illustrates an example of elements of a power receiver inaccordance with some embodiments of the invention;

FIG. 9 illustrates an example of elements of a power receiver inaccordance with some embodiments of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following description focuses on embodiments of the inventionapplicable to a wireless power transfer system utilizing a powertransfer approach such as known from the Qi specification. However, itwill be appreciated that the invention is not limited to thisapplication but may be applied to many other wireless power transfersystems.

FIG. 1 illustrates an example of a power transfer system in accordancewith some embodiments of the invention. The power transfer systemcomprises a power transmitter 101 which includes (or is coupled to) atransmitter coil/inductor 103. The system further comprises a powerreceiver 105 which includes (or is coupled to) a receiver coil/inductor107.

The system provides an electromagnetic power transfer signal which mayinductively transfer power from the power transmitter 101 to the powerreceiver 105. Specifically, the power transmitter 101 generates anelectromagnetic signal, which is propagated as a magnetic flux by thetransmitter coil or inductor 103. The power transfer signal maycorrespond to the electromagnetic power transfer component representingthe energy transfer from the power transmitter to the power receiver,and may be considered to correspond to the component of the generatedelectromagnetic field that transfers power from the power transmitter tothe power receiver. For example, if there is no loading of the receivecoil 107, no power will be extracted by the power receiver from thegenerated electromagnetic field (apart from losses). In such a scenario,the driving of the transmitter coil 103 may generate an electromagneticfield of potentially high field strength but the power level of thepower transfer signal will be zero (apart from losses). In somesituations, where a foreign object is present, the power transfer signalmay be considered to include a component corresponding to the powertransfer to the foreign object, and thus the power transfer signal maybe considered to correspond to the power being extracted from theelectromagnetic field generated by the power transmitter.

The power transfer signal may typically have a frequency between around20 kHz to around 500 kHz, and often for Qi compatible systems typicallyin the range from 95 kHz to 205 kHz (or e.g. for high power kitchenapplications, the frequency may e.g. typically be in the range between20 kHz to 80 kHz). The transmitter coil 103 and the power receiving coil107 are loosely coupled and thus the power receiving coil 107 picks up(at least part of) the power transfer signal from the power transmitter101. Thus, the power is transferred from the power transmitter 101 tothe power receiver 105 via a wireless inductive coupling from thetransmitter coil 103 to the power receiving coil 107. The term powertransfer signal is mainly used to refer to the inductive signal/magneticfield between the transmitter coil 103 and the power receiving coil 107(the magnetic flux signal), but it will be appreciated that byequivalence it may also be considered and used as a reference to anelectrical signal provided to the transmitter coil 103 or picked up bythe power receiving coil 107.

In the example, the power receiver 105 is specifically a power receiverthat receives power via the receiver coil 107. However, in otherembodiments, the power receiver 105 may comprise a metallic element,such as a metallic heating element, in which case the power transfersignal directly induces eddy currents resulting in a direct heating ofthe element.

The system is arranged to transfer substantial power levels, andspecifically the power transmitter may support power levels in excess of500 mW, 1 W, 5 W, 50 W, 100 W or 500 W in many embodiments. For example,for Qi corresponding applications, the power transfers may typically bein the 1-5 W power range for low power applications (the basic powerprofile), up to 15 W for Qi specification version 1.2, in the range upto 100 W for higher power applications such as power tools, laptops,drones, robots etc., and in excess of 100 W and up to more than 1000 Wfor very high-power applications, such as e.g. kitchen applications.

In the following, the operation of the power transmitter 101 and thepower receiver 105 will be described with specific reference to anembodiment generally in accordance with the Qi Specification (except forthe herein described (or consequential) modifications and enhancements)or suitable for the higher power kitchen specification being developedby the Wireless Power Consortium. In particular, the power transmitter101 and the power receiver 105 may follow, or substantially becompatible with, elements of the Qi Specification version 1.0, 1.1 or1.2 (except for the herein described (or consequential) modificationsand enhancements).

In the following, the operation of the system of FIG. 1 will bedescribed with specific focus on foreign object detection.

In wireless power transfer systems, the presence of an object (typicallya conductive element extracting power from the power transfer signal andnot being part of the power transmitter 101 or the power receiver 105,i.e. being an unintended, undesired, and/or interfering element to thepower transfer) may be highly disadvantageous during a power transfer.Such an undesired object is in the field known as a foreign object.

A foreign object may not only reduce efficiency by adding a power lossto the operation but may also degrade the power transfer operationitself (e.g. by interfering with the power transfer efficiency orextracting power not directly controlled e.g. by the power transferloop). In addition, the induction of currents in the foreign object(specifically eddy currents in the metal part of a foreign object) mayresult in an often highly undesirable heating of the foreign object.

In order to address such scenarios, wireless power transfer systems suchas Qi include functionality for foreign object detection. Specifically,the power transmitter comprises functionality seeking to detect whethera foreign object is present. If so, the power transmitter may e.g.terminate the power transfer or reduce the maximum amount of power thatcan be transferred.

The inventors have realized that conventional foreign object detectionoperates suboptimally and that this is partly due to variations anduncertainties in the specific operating conditions and scenario in whichthe foreign object detection is performed, including variations anduncertainties in the power transmitter properties, power receiverproperties, test conditions applied etc.

An example of the challenges to foreign object detection tests is therequirement to perform sufficiently accurate measurements in order toachieve a sufficiently reliable foreign object detection. For example,if a measurement for a foreign object detection takes place in theselection phase of a Qi power transfer initialization phase, the signalthat the power transmitter provides for this measurement has to be smallenough not to wake up the power receiver. However, this typicallyresults in poor signal/noise ratios leading to reduced detectionaccuracy. Therefore, the detection performance may be sensitive to thespecific signal level applied and there will typically be conflictingrequirements.

A power receiver exposed to a small electromagnetic signal may show aleakage current that depends on the level of the electromagnetic signal,the coupling between the primary and secondary coil, and the chargingstate of the capacitor at the output of the rectifier. This leakagecurrent can therefore vary depending on the actual conditions currentlyexperienced and depending on the specific parameters (e.g. properties ofcapacitor) of the individual power receiver. Since leakage currentinfluences the reflected impedance at the primary coil, the measurementof the quality factor also depends on the actual conditions and thistypically prevents optimal detection.

Yet another problem detecting a foreign object based on e.g. reportedreceived power indications at different loads or signal levels can beless reliable than desired due to the relationships between transmittedand received power being different for different loads and signalslevels.

The system of FIG. 1 uses an approach for foreign object detection thatseeks to reduce uncertainty and sensitivity to variations, andaccordingly it seeks to provide improved foreign object detection. Theapproach may in many embodiments provide improved foreign objectdetection and specifically may in many embodiments provide a moreaccurate and/or reliable foreign object detection. The approach mayfurther allow low complexity and low resource requirements. An advantageof the approach is that it may be suitable for inclusion in manyexisting systems, such as specifically in a Qi wireless power transfersystem, and indeed that this may often be achieved with fewmodifications.

As will be described in more detail in the following, the approachutilizes a time division approach during the power transfer phasewherein operations, such as foreign object detection, and power transfermay e.g. be performed in different time intervals thereby allowing theinterference between these (specifically the impact of the powertransfer on the foreign object detection) to be reduced substantially.

Specifically, for the wireless power transfer system, the power transfersignal is subject to a repeating time frame which comprises at least onepower transfer time interval and one foreign object detection timeinterval.

The power transmitter is arranged to perform foreign object detectionduring the foreign object detection time intervals and in order tofacilitate this operation, the power receiver is arranged to disconnecta load thereby reducing the loading of the power transfer signalcompared to during power transfer intervals for which the load isconnected such that it loads the power transfer signal.

In particular, many foreign object detection approaches become moreaccurate if the power received/extracted by the power receiver isreduced to (close to) zero by disconnecting the load. In this condition,if power is provided by the power transmitter it is likely to mainly beabsorbed by a foreign object in proximity of the power transmitter. Suchpower being extracted in a foreign object can be measured and detectedwith a much higher accuracy because the power level and uncertainty ofthe power extracted by the power receiver is reduced substantially andtypically will be reduced to e.g. only that extracted by friendly metalof the power receiver. Not only does this increase the relative impactof the foreign object on the power transfer signal but it may alsotypically enable a more accurate compensation for the power extracted bythe power receiver (e.g. the power extracted by friendly metal can beestimated during a calibration process and subsequently compensated forin the foreign object detection algorithm).

The disconnection of the load during the foreign object detection timeintervals results in the power level of the power transfer signal beingreduced during the foreign object detection time interval relative tothe power transfer time interval, and typically the power extracted bythe power receiver may be no less than 5, 10, or 50 times lower than thepower level during the power transfer time interval.

The power transmitter thus performs the foreign object detection duringthe foreign object detection time intervals when the loading of thepower transfer signal by the power receiver is substantially reduced.Further, the power receiver synchronizes to the repeating time frame onthe power transfer signal and uses this to synchronize the disconnectionof the load such that this aligns with the foreign object detection timeintervals. Typically the load will be disconnected shortly after thedetermined start of the foreign object detection time interval andreconnected shortly before the end of it thereby providing a smallsafety margin. As a result of the time frame synchronized operations ofthe power transmitter and the power receiver, improved foreign objectdetection is performed while still allowing a high transferred powerlevel.

FIG. 2 illustrates elements of the power transmitter 101 and FIG. 3illustrates elements of the power receiver 105 of FIG. 1 in more detail.

The power transmitter 101 includes a driver 201 which can generate adrive signal that is fed to the transmitter coil 103 which in returngenerates an electromagnetic field and thus the electromagnetic powertransfer signal which provides power transfer to the power receiver 105.The power transfer signal is provided (at least) during power transfertime intervals of the power transfer phase.

The driver 201 may typically comprise an output circuit in the form ofan inverter, typically formed by driving a full or half bridge as willbe well known to the skilled person. FIG. 4 illustrates an example of atypical output stage of a power transmitter where an inverter is formedby four FETs coupled in a bridge configuration and with the transmittercoil 103 (LTx) further being coupled to a capacitor (CTx) to form aresonant output circuit.

The power transmitter 101 further comprises a power transmittercontroller 203 which is arranged to control the operation of the powertransmitter 101 in accordance with the desired operating principles.Specifically, the power transmitter 101 may include many of thefunctionalities required to perform power control in accordance with theQi Specifications.

The power transmitter controller 203 is in particular arranged tocontrol the generation of the drive signal by the driver 201, and it canspecifically control the power level of the drive signal, andaccordingly the level of the generated power transfersignal/electromagnetic field. The power transmitter controller 203comprises a power loop controller controlling a power level of the powertransfer signal in response to power control messages received from thepower receiver 105 during the power control phase.

In order to receive data and messages from the power receiver 105, thepower transmitter 101 comprises a first communicator 205 which isarranged to receive data and messages from the power receiver 105 (aswill be appreciated by the skilled person, a data message may provideone or more bits of information). In the example, the power receiver 105is arranged to load modulate the power transfer signal generated by thetransmitter coil 103, and the first communicator 205 is arranged tosense variations in the voltage and/or current of the transmitter coil103 and to demodulate the load modulation based on these. The skilledperson will be aware of the principles of load modulation, as e.g. usedin Qi wireless power transfer systems, and therefore these will not bedescribed in further detail.

In many embodiments, the first communicator 205 is further arranged totransmit data to the power receiver 105 and may specifically be arrangedto modulate the power transfer signal using frequency, amplitude, orphase modulation.

In some embodiments, communication may be performed using a separatecommunication channel which may be achieved using a separatecommunication coil, or indeed using the transmitter coil 103. Forexample, in some embodiments Near Field Communication may be implementedor a high frequency carrier (e.g. with a carrier frequency of 13.56 MHz)may be overlaid on the power transfer signal.

In the system of FIGS. 1-3, the communication is during the powertransfer phase performed in foreign object detection time intervals.Specifically, some or indeed all of the foreign object detection timeintervals may also be used as communication time intervals in whichcommunication between the power transmitter 101 and the power receiver105 is performed. Specifically, the transmitter controller 203 maysynchronize the first communicator 205 such that the communicationoperation (typically both receiving and transmitting data) is performedin (and typically only in) the foreign object detection timeintervals/communication time intervals of the power transfer phase, i.e.in the foreign object detection time intervals that are assigned forcommunication.

This may substantially improve communication performance.

The power transmitter 101 further comprises a foreign object detector207 which is arranged to perform foreign object detection tests, i.e. tospecifically detect whether any undesired conductive elements are likelyto be present within the generated electromagnetic field.

In the system, the foreign object detection tests are based onmeasurements performed during foreign object detection time intervals,i.e. during foreign object detection time intervals that are assigned toforeign object detection.

As will be described later in more detail, during the foreign objectdetection time intervals, the power level of the power transfer signalis reduced by the power receiver disconnecting its load and reducing theoverall loading of the power transfer signal. The disconnection of theload corresponds to decoupling the load from the power transfer signal(and this the terms disconnecting and decoupling the load from the powertransfer signal may be considered synonymous). The load will accordinglybe decoupled/disconnected from the power transfer signal, and from thereceive coil 107.

In many embodiments, the power receiver 105 may be arranged to minimizethe loading of the power transfer signal to only correspond to loadingresulting from friendly metal (metal parts of the power receiver itself)and possible a small amount of power used by control functionality ofthe power receiver. The power receiver may often completely disconnectthe target load from the power transfer signal during the foreign objectdetection time intervals. This may for example often reduce the loadingof the power transfer signal from e.g. 5-50 W during the power transfertime intervals to less than 500 mW during the foreign object detectiontime intervals.

It should be noted that the power level of the power transfer signal maybe reduced without this resulting in (or being caused by) a reduction inthe generated electromagnetic field strength. For example, the powerreceiver disconnecting the load will result in a reduced amount of powerbeing extracted from the electromagnetic field and the power transfersignal, and thus from the drive signal to the transmitter coil 103.However, this needs not result in reduction in the generated fieldstrength and indeed may result in a large field strength as the opposingelectromagnetic field caused by the current in the receiver coil 107 isreduced.

Thus, in many embodiments, the foreign object detection time intervalsare characterized by a reduced power transfer from the power transmitterto the power receiver in comparison to that during the power transfertime intervals (or at least by a reduced maximum possible/availablepower transfer from the power transmitter to the power receiver incomparison to the maximum possible/available power transfer during thepower transfer time intervals). However, the strength of theelectromagnetic field generated by the transmitter coil 103 may remainthe same or even increase.

Indeed, in many embodiments where the foreign object detection is basedon measuring the loading of the electromagnetic field generated by thetransmitter coil 103, it may be desirable to adapt the drive signal suchthat the generated electromagnetic field has a field strength suitablefor performing the intended operation during the foreign objectdetection time interval. This may possibly even be a higher fieldstrength than during the power transfer time interval but the amount ofpower being transferred is reduced due to the power receiverdisconnecting the load. In most implementations, it is however desirablefor the field strength to not be too high when the receiver disconnectsthe load (in order to mitigate against overvoltage conditions).

The reduced loading allows for a much more accurate foreign objectdetection in many situations. It will result in the power dissipated ina foreign object being a much larger proportion of the total powerdissipation, and indeed typically in the foreign object dissipated powerexceeded the power dissipated in the power receiver, thereby making thedetection of this foreign object power dissipation much easier.

In the approach of FIG. 2, the electromagnetic test signal is generatedby the driver 201 driving the transmitter coil and thus theelectromagnetic test signal may be considered to correspond to the powertransfer signal during the foreign object detection time intervals.However, it will be appreciated that in some embodiments, theelectromagnetic test signal may be generated by a different coil thanthe transmitter coil 103 (e.g. a dedicated test coil). In the following,the term power transfer signal will be used to refer to theelectromagnetic field signal generated by the power transmitter duringpower transfer time intervals and during foreign object detection timeintervals.

During an interval in which foreign object detection is performed, i.e.during a foreign object detection time interval, the foreign objectdetector 207 may evaluate conditions to determine whether a foreignobject is considered present or not. During the foreign object detectiontime interval, the power transmitter 101 generates an electromagnetictest signal and the foreign object detection is based on evaluatingcharacteristics and properties of this signal.

For example, the power level of (the power extracted from) the generatedpower transfer signal may be used as an indication of the power beingextracted by potential foreign objects (typically by comparing it to theexpected power extraction from the power receiver 105). The power levelof the power transfer signal reflects the power that is extracted fromthe generated electromagnetic field by conductive elements (includingthe receiver coil 107) present in the electromagnetic field. It thusindicates the power extracted by the combination of the power receiver105 as well as any foreign objects that may be present. The differencebetween the power level of the power transfer signal and the powerextracted by the power receiver 105 accordingly reflects the powerextracted by any foreign objects present. The foreign object detectionmay for example be a low complexity detection wherein a detection of aforeign object is considered to have occurred if the difference betweenthe power level of the electromagnetic signal (henceforth referred to astransmit power level) exceeds the reported power extracted by the powerreceiver 105 (henceforth referred to as received power level).

In the approach, the foreign object detection is accordingly based on apower level comparison between a transmitted power level and a reportedreceived power level. The reaction to a detection of a foreign objectmay be different in different embodiments. However, in many embodiments,the power transmitter 101 may be arranged to terminate a power transfer(at least temporarily) in response to a detection of a foreign object.In other embodiments, it may be arranged to impose reduced power limitson the power transfer signal during the power transfer time intervalswhile allowing power transfer to proceed.

FIG. 3 illustrates some exemplary elements of the power receiver 105.

The receiver coil 107 is coupled to a power receiver controller 301which couples the receiver coil 107 to a load 303 via a switch 305 (i.e.it is a switchable load 305). The power receiver controller 301 includesa power control path which converts the power extracted by the receivercoil 107 into a suitable supply for the load. In addition, the powerreceiver controller 301 may include various power receiver controllerfunctionality required to perform power transfer, and in particularfunctions required to perform power transfer in accordance with the Qispecifications.

In order to support communication from the power receiver 105 to thepower transmitter 101 the power receiver 105 comprises a secondcommunicator 307.

The second communicator 307 is arranged to transmit data to the powertransmitter by varying the loading of the receiver coil 107 in responseto data to be transmitted to the power transmitter 101. The loadvariations are then detected and demodulated by the power transmitter101 as will be known to the person skilled in the art.

In the example, the second communicator 307 is furthermore arranged todemodulate amplitude, frequency, and/or phase modulation of the powertransfer signal in order to retrieve data transmitted from the powertransmitter.

The power receiver controller 301 is further arranged to control thesecond communicator 307 such that the communication during the powertransfer phase is performed in the communication intervals, i.e. duringtime intervals in which the power level of the power transfer signal isreduced.

Thus, similarly to the first communicator synchronizing communicationwith the power receiver to occur during foreign object detection timeintervals, the second communicator also synchronizes communication withthe power transmitter to occur during foreign object detection timeintervals.

FIG. 5 illustrates a circuit diagram of elements of an example of apower path of the power receiver 105. In the example, the power receiver105 comprises the receiver coil 107 referred to by the designation LRX.In the example, receiver coil 107 is part of a resonance circuit and thepower receiver 105 accordingly also includes a resonance capacitor CRX.The receiver coil 107 is subjected to the electromagnetic signal/fieldand accordingly an AC voltage/current is induced in the coil. Theresonance circuit is coupled to a rectifier bridge with a smoothingcapacitor C1 coupled to the output of the bridge. Thus, a DC voltage isgenerated over the capacitor C1. The magnitude of the ripple on the DCvoltage will depend on the size of the smoothing capacitor as well as onthe load.

The bridge B1 and smoothing capacitor C1 are coupled to the load 303which is indicated by reference sign RL via the switch 305 which isillustrated by switch S1. The switch 305 can accordingly be used toconnect or disconnect the load from the power path and thus the load isa switchable load 305. It will be appreciated that whereas the switch S1is shown as a conventional switch, it may of course be implemented byany suitable means including typically by a MOSFET. It will also beappreciated that the load 303 is illustrated as a simple passive portbut that it may of course be any suitable load. For example, the load303 may be a battery to be charged, a mobile phone, or anothercommunication or computational device, may be a simple passive load etc.Indeed, the load 303 need not be an external or dedicated internal loadbut may for example include elements of the power receiver 105 itself.Thus, the load 303 illustrated in FIGS. 3 and 5 may be considered torepresent any load of the receiver coil 107/the electromagnetic signalthat can be disconnected by the switch 305/S1, and it is accordinglyalso referred to as a switchable load 305.

FIG. 5 further illustrates a load modulation capacitor C2 which can beconnected or disconnected in parallel to the resonance circuit based onthe switching of switch S2. The second communicator 307 may be arrangedto control the switch S2 such that the load of the modulation capacitorC2 can be connected and disconnected in response to data to betransmitted to the power transmitter 101 thereby providing loadmodulation.

The power receiver 105 is arranged to enter a reduced power mode duringthe foreign object detection time interval(s) of each time frame duringthe power transfer phase. In the example, the power receiver 105comprises a load controller 309 which controls the switch 305(equivalently the switch 305 can be considered part of the loadcontroller). During a foreign object detection time interval, the loadcontroller 309 can disconnect the load 303 from the power receiver, i.e.it disconnects a load of the power receiver controller 301, and thus aload of the receiver coil 107. Thus, in this way the load controller 309may reduce the loading of the receiver coil 107 during the foreignobject detection time interval. Furthermore, not only is the load of thepower receiver 105 reduced thereby making it easier to detect otherpower loss or to detect modulation but often more importantly the powerreceiver 105 enters a more well-defined or certain state in which theimpact of load variations on the power transfer signal is reduced.

In the example of FIG. 5, the switch S1 may be used to disconnect theload during the foreign object detection time interval. It will beappreciated that in embodiments where the switchable load 303 requires amore constant power provision, the switch S1 may be positioned beforethe capacitor C1 or another energy reservoir may be provided afterswitch S1 to supply the switchable load 303 with power during theforeign object detection time interval.

It will be appreciated that the loading of the receiver coil 107 may notbe completely switched off during the foreign object detection interval.For example, the power receiver 105 may still extract power for e.g.operating some internal circuitry. Thus, the load controller 309 may bearranged to disconnect a load from loading the receiver coil 107 whilestill allowing the receiver coil 107 to be loaded by one or more otherloads. Indeed, the loading of the receiver coil 107 can be considered asbeing comprised of a load which is disconnected by the load controller309 during the foreign object detection interval and a load which is notdisconnected by the load controller 309. Thus, the load 303 can beconsidered to represent the load that is disconnected by the receivercoil 107 during the foreign object detection interval. This load mayinclude both an external or internal load for which the power transferis established but may also include for example internal controlfunctionality temporarily switched off during the foreign objectdetection interval.

The power receiver controller 301 is arranged to establish a powercontrol loop with the power transmitter 101. Specifically, the powerreceiver controller 301 can transmit power control messages to the powertransmitter 101 and in response the power transmitter 101 may change thepower level of the power transfer signal during the power transfer timeintervals. Typically, the power receiver controller 301 may generatepower control error messages which indicate a request for the powertransmitter 101 to increase or decrease the power level. The powerreceiver controller 301 may determine the appropriate error messages bycomparing a measured value to a reference value. During power transfer,the power receiver controller 301 may compare the provided power levelwith the required power level and request an increased or decreasedpower level based on this comparison.

As previously mentioned, the system applies a repeating time frameduring the power transfer phase where the time frame comprises at leastone power transfer time interval and one foreign object detection timeinterval. An example of such a repeating time frame is illustrated inFIG. 6 where power transfer time intervals are indicated by PT andforeign object detection time intervals are indicated by D. In theexample, each time frame FRM comprises only one foreign object detectiontime interval and one power transfer time interval. However, it will beappreciated that in other embodiments, other time intervals may also beincluded in a time frame or a plurality of foreign object detection timeintervals and/or power transfer time intervals may be included in eachtime frame. Specifically, a repeating time frame may comprise differenttypes of foreign object detection time intervals, such as one or moreforeign object detection time intervals and one or more communicationtime intervals.

In the approach, foreign object detection (and e.g. operations such ascommunication) may be performed in the foreign object detection timeintervals, and thus the foreign object detection (and e.g.communication) and the power transfer may be separated in the timedomain thereby resulting in reduced cross-interference from the powertransfer to the foreign object detection/communication. Thus, thevariability and uncertainty resulting from variations in the operatingconditions for the power transfer can be isolated from the foreignobject detection (communication) resulting in a more reliable andaccurate foreign object detection (communication).

In the power transfer phase, the power transmitter is thus arranged toperform power transfer during the power transfer time interval of thetime frames of the power transfer phase. Specifically, during these timeintervals, the power transmitter and the power receiver may operate apower control loop (the power control loop may be based on communicationwithin communication time intervals corresponding to repeating timeintervals). Thus, the level of the power being transferred may bedynamically varied.

In the foreign object detection time intervals of the time frames of thepower transfer phase, the power receiver disconnects/decouples a loadresulting in a reduction in the power level transferred to the powerreceiver by the power transfer signal.

As previously mentioned, the reduction in the power level need notcorrespond to a reduction of the field strength of the generatedelectromagnetic field. For example, when the power transfer level issignificantly reduced by the loading of the field by the power receiverbeing reduced, e.g. by disconnecting the load 303, the resultingelectromagnetic field strength/the signal level of the generatedelectromagnetic signal may increase. Indeed, it may typically bedesirable to keep the magnetic field strength relatively high in orderto measure sufficient power dissipation in a foreign object, and thus inorder to facilitate and improve foreign object detection. At the sametime, it is typically desirable that the field strength is sufficientlylow to not cause unacceptable overvoltage conditions when the load 303is disconnected.

The power receiver 105 may accordingly reduce a loading by the powerreceiver of the power transfer signal during a foreign object detectiontime interval. Specifically, the load of the power transfer signal(functioning as an electromagnetic test signal) by the power receiverduring the foreign object detection time interval will be less than theload of the power transfer signal by the power receiver during the powertransfer time interval (the load may e.g. be considered the effectiveresistive impedance of respectively the transmitter coil 103 and thetest coil 209 during the power transfer time interval and the foreignobject detection time interval respectively).

By reducing the level of power extracted by the power receiver, thepresence of any foreign object becomes easier to detect. This may resultfrom the total amount of extracted power being reduced thereby making iteasier to detect lower power levels for power extracted by a foreignobject. Further, by disconnecting the load, the power receiver maytypically be entered into a predetermined (or at least easier topredict) state thereby facilitating compensation for the power receiverwhen performing the foreign object detection test.

Thus, the disconnection of the switchable load 303 not only reduces theload of the power transfer signal but may also provide for this load tobe more predictable and to have reduced variation. Typically, the loadof a power transmitter by a power receiver may vary substantially notonly from application to application, but also as a function of time forthe same application and power transfer session. The power control loopis operated during the power transfer phase to adapt to such variations.However, by introducing a foreign object detection time interval inwhich the load may be disconnected, it is possible to enter the powerreceiver into a reference mode in which the loading of theelectromagnetic field is more predictable. Thus, e.g. the foreign objectdetection tests can be performed based on the assumption that the powerreceiver is in this reference or test mode, and thus e.g. apredetermined loading of the electromagnetic test signal can be assumed.The approach may thus not only allow for the loading by the powerreceiver 105 to be reduced (thereby improving accuracy by the relativeimpact of any foreign objects being higher) but also allows this to bemore predictable thereby facilitating the compensation for the presenceof the power receiver during the foreign object detection test.

Thus, the system of FIGS. 1-5 provides for a much improved foreignobject detection test approach where the foreign object detection testsare performed under much more controlled conditions thereby allowing amore accurate and reliable foreign object detection tests to beperformed.

A significant issue in such a system is that it is important for thepower receiver and the power transmitter to be closely synchronized, andit is specifically important that the power receiver connects and inparticularly disconnects the load at the appropriate times. For example,during a typical power transfer operation, the power level of the powertransfer signal may be very high during the power transfer timeintervals. If the load of the power receiver is disconnected during atime when the power transmitter generates such a strong magnetic fieldand power transfer signal, an overvoltage may be induced in the powerreceiver when the load is disconnected. When the load is disconnected,the resonant circuit of the power transmitter, which is coupled via thetransmitter coil (103) and the receiver coil (107) of the powerreceiver, is no longer damped by the load. Indeed, there is a risk thatthe induced voltage at the power receiver may increase to a level thatmay cause damage to components of the power receiver when it disconnectsthe load.

However, the repeating time frame enables the power transmitter toreduce the signal level during the foreign object detection timeintervals to a level at which the induced voltage does not harm thecomponents of the power receiver. If the power receiver disconnects itsload after an appropriate reduction in the electromagnetic fieldstrength/signal level, the overvoltage condition at disconnection can beavoided. For example, the power receiver may measure e.g. the inducedvoltage at its receiver coil to detect a reduction of the power signalin order to use this as a trigger for the start of a foreign objectdetection interval and it may proceed to disconnect its load on thistrigger. However, this is often not a very reliable method, because thereduction may have been caused by other conditions than as an indicationof the start of a foreign object detection interval. For example, thepower transmitter may reduce the power signal due to a control errorpreviously received from the power receiver. Also, a change in the loadmay lead to a reduction of the induced Voltage at the receive coil. Thepower receiver therefore needs a reliable method for determining thestart of a foreign object detection interval in situations where thepower signal is high in the power transfer intervals to preventovervoltage and damage to its components when disconnecting the load.

At the end of the foreign object detection time interval, the load isreconnected to the receive coil 107 such that it loads the powertransfer signal. Again, it is desirable for the load to be connectedprior to the power transfer time interval in order to ensure that theincreased electromagnetic field does not induce too high a voltageresulting in an overvoltage condition. Again, the power receiver needs areliable method to re-connect its load in time, namely before the powertransmitter increases the signal level.

Thus, it is typically advantageous for the disconnection and there-connection of the load to occur within the foreign object detectiontime interval. However, it is desirable for the time available forforeign object detection to be sufficiently large to allow accuratemeasurement (with sufficient averaging). Furthermore, these conflictingpreferences are constrained by the overall desire to make foreign objectdetection time intervals as short as possible in order to increaseefficiency and minimize interruptions in the effective power transfer.Therefore, it is desirable to disconnect the load as shortly after thestart of the foreign object detection time interval as possible and toconnect the load as close to the end of the foreign object detectiontime interval as possible. In order to achieve safe disconnection of theload and optimal performance, it is important to reliably and closelysynchronize the operation of the power receiver, and in particular theload controller 309, to the repeating time frame.

The timing of the operations of the power receiver is based on a localtime reference/time base and the load controller 309 is arranged to timethe connection and the disconnection of the load 303 based on the localtime reference/base. Further, the power receiver 105 comprises asynchronizer 311 which is arranged to synchronize the local timereference to the repeating time frame. Thus, the power receiver 105comprises functionality for synchronizing the connecting anddisconnecting of the load 303 to the repeating time frame of the powertransfer signal.

For example, the local time reference may be implemented using atimer/clock etc. as will be well known to the skilled person. In someembodiments, this timer/clock may be a free running time base and thesynchronizer 311 may determine times of this free running time base thatcorresponds to the different time slots/intervals of the repeating timeframe. For example, the synchronizer 311 may determine which time valuesof the time base correspond to the beginning and end of the foreignobject detection time intervals, and the connecting and disconnecting ofthe load 303 may then be timed to occur at these times (typically with asmall offset).

In other embodiments, the synchronizer 311 may be arranged to varyparameters or settings of the time reference such that this aligns withthe repeating time frame. For example, a countdown timer may be startedwhen the synchronizer 311 considers a foreign object detection timeinterval to begin (or just after) and may have a duration determined tocorrespond to the duration of the foreign object detection time intervalas determined by the synchronizer 311 (or slightly shorter). The loadcontroller 309 may then be arranged to disconnect the load 303 when thecountdown timer is started and reconnect it when the countdown timer hasfinished the countdown.

In many embodiments, the synchronizer 311 may be arranged to implement atiming loop with an error signal being generated to reflect differencesbetween the repeating time frame and the timing loop. The timing loopmay then be driven by this error signal thereby resulting in a biastowards a minimization of the timing error/difference and thus resultingin the timing loop being synchronized with the repeating time frame. Forexample, a timing phase locked loop may be implemented to generate alocal time clock that is synchronized to the repeating time frame andspecifically to the foreign object detection time intervals.

It will be appreciated that the specific approach for synchronizing thelocal time reference to the repeating time frame may depend on thespecific preferences and requirements of the individual embodiment.

For example, in some embodiments, the input to the inverter driving thetransmitter coil 103 may generate a drive signal having dynamicvariations that may be synchronized with the repeating time frame (forexample the inverter may be fed with a periodically varying power supplyvoltage (e.g. generated by rectification of an AC supply voltage)). Thismay result in periodic and synchronized variations in the power transfersignal that the synchronizer 311 can detect and synchronize to.

In many power transmitters, however, such periodic variations in thedrive signal/power transfer signal are not practical or desirable, e.g.typically a smoothed and regulated DC voltage is used as a supplyvoltage to the inverter. In such embodiments, the power transmitter maybe arranged to vary properties of the power transfer signal/generatedelectromagnetic field/signal between power transfer intervals andforeign object detection time intervals, and the synchronizer 311 may bearranged to synchronize the local time reference based on thesevariations.

The synchronizer 311 thus specifically enables the timing of theconnection and disconnection of the load 303 to be closely synchronizedto the repeating time frame and the foreign object detection timeintervals. It may specifically allow the disconnection to occur shortlyafter the start of a foreign object detection time interval and for thereconnection to occur shortly before the end of a foreign objectdetection time interval thereby ensuring that the duration of theforeign object detection time interval can be reduced and the overheadand margin of ensuring reliable operation when connecting anddisconnecting a load can be minimized.

However, in the power receiver of FIG. 3, the synchronizer 311 does notonly perform the synchronization but it also proceeds to generate asynchronization reliability measure. The reliability measure isgenerated as an indication of the estimated reliability/accuracy of thesynchronization. It may specifically be considered to reflect howclosely the time reference is synchronized to the repeating time frameand/or the probability that the synchronization is within a givensynchronization window. The reliability measure may be generated toprovide an estimate of the difference between the synchronized timereference and the repeating time frame. The estimate may be aprobabilistic estimate.

It will be appreciated that whereas a number of different approacheswill be described for determining a reliability measure, the specificapproach will depend on the preferences and requirements of theindividual embodiment, and that many more or less accurate approachescan be used by the skilled person.

The power receiver 105 further comprises a mode controller 313 which isfed the determined reliability measure for the synchronization. The modecontroller 313 is arranged to switch the power receiver 105 betweendifferent operational modes, and specifically is arranged to switch thepower receiver between a first operational mode and a second operationalmode for the power transfer time intervals in response to thereliability measure. Thus, the power receiver 105 is arranged to operatedifferently during the power transfer time intervals depending on thereliability of the synchronization of the power receiver 105, andspecifically on the synchronization of the disconnection of the load, tothe repeating time frame.

In many embodiments, the level of the power transfer signal during thepower transfer time intervals is reduced when in the first mode relativeto when in the second mode of operation.

The synchronization performance is accordingly taken into account forthe operation of the power receiver, and indeed the operation is adaptedduring the power transfer time intervals dependent on the reliability ofthe synchronization of the disconnection (and typically thereconnection) of the load—even if these actions occur during the foreignobject detection time intervals.

The first and second operational modes may specifically employ differentpower transfer parameters for the power transfer operation. For example,the power levels requested, and/or the loading and power extracted fromthe power transfer signal may be different in the first and secondoperational modes. The first and second operational modes may employdifferent power transfer parameters by at least one of: applying adifferent limit for a signal level of the power transfer signal in thefirst and second operational modes; employing a different load of thepower transfer signal in the first and second operational modes;applying a different limit to rate of changes for power levels of thepower transfer signal in the first and second operational modes. Theseexamples will be described in more detail later.

The mode controller 313 may specifically be arranged to switch the powerreceiver 105 from the first mode (of operation) to the second mode (ofoperation) in response to a detection that the reliability measureincreases above a given threshold. Similarly, the mode controller 313may be arranged to switch the power receiver 105 from the second mode(of operation) to the first mode (of operation) in response to adetection that the reliability measure falls below a given threshold(the two thresholds may be the same but are not necessarily so). Thus,the power receiver 105 may operate in different modes during the powertransfer time intervals depending on whether the synchronization isreliable or not.

This may provide a more reliable and more safe operation in manyembodiments.

For example, in some embodiments, the power receiver 105 andspecifically the load controller 309 are arranged to disconnect the load303 during the power transfer time intervals when in the first mode butnot when in the second mode. Indeed, in this example, the load 303 mayin some embodiments be disconnected during the entire repeating timeframe when in the first mode whereas periodic disconnection of the load303 during the foreign object detection time intervals is employed whenoperating in the second mode. Accordingly, when the reliability measureindicates that the synchronization is not reliable, the load 303 may bedisconnected during both power transfer time intervals and foreignobject detection time intervals, and typically for the whole duration ofthe repeating time frame whereas when the reliability measure indicatesthat the synchronization is reliable, the load 303 is only disconnectedtemporarily during the foreign object detection time intervals.

In addition, in order to prevent an unacceptably high induced voltage atthe power receiver, the power receiver may ensure that the signal levelprovided by the power transmitter is sufficiently low. The powerreceiver can control the level of the power signal provided by the powertransmitter by sending control errors. Specifically, if thesynchronization is considered to not be reliable, the power receiver maycontrol the power transmitter to reduce the power level/signal strength.

In this approach, the first mode ensures that there are no disconnectionswitchings of the load during times at which the induced signal maypotentially have a large value. Thus, errors or inaccuracies in thesynchronization will not result in overvoltage conditions occurring bydisconnections during the power transfer time intervals. However, whenthe synchronization is sufficiently accurate to ensure that thedisconnections occur within foreign object detection time intervals, thesystem moves to an operational mode wherein the power transfer isperformed (the signal level provided by the power transmitter may becontrolled to a high level by the power receiver) and the load connectedduring the foreign object detection time intervals. This shift betweendifferent operational modes may allow the design parameters and criteriafor the foreign object detection time intervals and the loaddisconnection to be much more reliable in the second mode of operation,while also allowing much tighter and with less margin. This prevents anovervoltage condition that may cause damage to the electrical componentsof the power receiver and may reduce the time required for foreignobject detection time intervals to be reduced as less margin is requiredfrom the start of a foreign object detection time interval to the loaddisconnection.

Such an approach may in some embodiments be applied throughout the powertransfer phase and e.g. the power receiver may continuously monitor thestatus of the synchronization and switch to the first mode if thereliability measure becomes indicative of the synchronization not beingsufficiently reliable, and then switch back to normal operation when thereliability measure indicates that the synchronization is againreliable.

This may be suitable for some applications, such as non-critical batterycharging but may be unsuitable for other applications wherein aguaranteed continuous power transfer performance is required. In suchscenarios, the power receiver may e.g. be arranged to terminate thepower transfer operation completely if the synchronization becomesunreliable. Further, in such embodiments, the parameters may bedetermined to ensure that such a situation occurs only very rarely.

In many embodiments, the system may be arranged to adopt asynchronization phase initially when entering the power transfer phase.Thus, the power transfer phase may start with a synchronization intervalin which synchronization of the power receiver to the reduced power timeinterval of the power transfer signal is performed. In such embodiments,the power receiver may enter the power transfer phase and thesynchronization phase/time interval in the first mode of operation andonly switch to the second mode when the reliability measure indicatesthat the synchronization is sufficiently reliable. Such an approach willtypically be advantageous even for more critical applications requiringa continuous power transfer.

Thus, in many embodiments, the mode controller 313 controls the powerreceiver 105 to operate in the first operational mode when it enters thepower transfer phase. Further, when entering the power transfer phase,the synchronizer 311 performs synchronization of the local timereference to the repeating time frame. Thus, the system may start thepower transfer phase in a state and mode wherein the power receiver 105is not (guaranteed to be sufficiently) synchronized to the repeatingtime frame. However, this is compensated by the power receiver 105operating in the first mode which may be designed to provide appropriateperformance for the situation when the power receiver 105 is notsynchronized to the power transmitter 101/the repeating time frame. Inthe specific example, this is achieved by keeping the load 303permanently disconnected throughout the repeating time frame therebyensuring that overvoltage conditions caused by the disconnection of theload 303 cannot occur.

Additionally, the signal level provided by the power transmitter undercontrol of the power receiver is typically reduced in the first mode inorder to ensure that no overvoltage occurs while the load is permanentlydisconnected.

Typically, the operation is such that generated signal level and loadreductions o hand-in-hand:

1. A high signal level and disconnected load can lead to an overvoltageand even potential damage of the power receiver

2. A low signal level and connected load can lead to an undervoltage andcan stop the power receiver working.

Accordingly, the power receiver may modify the operation of both theconnection/disconnection of loads and the signal level setting based onwhich mode the power receiver is operating in.

In the specific example, a safe initial phase is entered in which thesynchronization may be performed (or a more accurate synchronization maybe achieved) and during which the power receiver is arranged to operatein a “safe” mode where synchronization errors do not result inpotentially damaging conditions arising. The power receiver in thespecific example disconnects the load 303 for the entire synchronizationphase and further controls the power transmitter to generate anelectromagnetic field with a signal level that is sufficiently low.

The mode controller 313 maintains the power receiver 105 in this first(safe) mode until the reliability measure indicates that thesynchronization is sufficiently accurate at which time it switches themode controller 313 to the second mode of operation in which “normal”power transfer proceeds, and specifically in which the load 303 isconnected during the power transfer time intervals. Thus, the powerreceiver 105 remains in the “safe” synchronization mode until thereliability measure indicates that sufficiently accurate and reliablesynchronization has been achieved.

It will be appreciated that whereas in the example above the load 303 isdisconnected completely during the synchronization phase, this is notnecessarily the case in other embodiments. For example, in someembodiments, only part of the load 303 may be disconnected during thesynchronization phase, i.e. the power receiver 105 may be arranged tocouple a lighter load to the receive coil 107 during the synchronizationphase than during normal power transfer. For example, the load may bereduced to a level wherein the overvoltage condition is acceptable.

Different approaches and algorithms may be used to synchronize the localtime reference to the repeating time frame in different embodiments. Inmany embodiments, the power transmitter 101 may be arranged to vary thelevel of the generated electromagnetic field such that the variationsare synchronized to the repeating time frame. The synchronizer 311 maythen monitor the level variations and synchronize the time reference tothis.

As an example, the power transmitter 101 may be arranged to insert ashort, predetermined pattern or signature of level variations at thestart of each repeating time frame and the synchronizer 311 may bearranged to detect this pattern/signature and determine the start timeof the repeating time frame from the timing of the detectedpattern/signature.

In many embodiments, the power transmitter 101 may be arranged togenerate the electromagnetic signal such that the signal level isdifferent in the power transfer time intervals and the foreign objectdetection time intervals and the synchronizer 311 may be arranged tosynchronize the local time reference to the resulting signal levelvariations. Specifically, the synchronizer 311 may detect the timing ofthe level transitions of the power transfer signal (specifically thesignal induced in the power receiver coil) and adjust or compensate thelocal time reference such that the locally generated repeating timeframe timing corresponds to these transitions. For example, in someembodiments, the time difference between the detected signal leveltransitions and the expected transition between a foreign objectdetection time interval and a power transfer time interval according tothe local time reference may be used as an error signal for a phasedlocked timing loop controlling the local time reference.

In some embodiments, the power transmitter may be arranged to introduceor ensure such transitions autonomously. However, in other embodiments,the power receiver may be arranged to communicate with the powertransmitter in order to ensure such signal level variations.

For example, the power receiver controller 301 may be arranged totransmit signal level requests for the power transfer signal to thepower transmitter during the synchronization phase, and thus while thepower receiver is operating in the first mode. These signal levelrequests may be selected such that the signal level of the powertransfer signal during the power transfer intervals differ from a signallevel of the power transfer signal during the foreign object detectionintervals. It may further control the power transmitter to generate asignal level which is sufficiently low to not generate unacceptableovervoltage conditions.

Specifically, the power transmitter 101 may be arranged to provide agiven signal level during the foreign object detection time intervals,such as e.g. a predetermined level or a level determined previously incommunication with the power receiver (as will be described later).Thus, the power transmitter may be arranged to set the signal level forthe drive signal, and thus for the generated electromagnetic field, tothis static value during the foreign object detection time intervals ofthe frames. However, the system may operate a power control loop todynamically adjust the levels of the power transfer time intervals. Thispower control loop may be active throughout the power transfer phase andbe used to adapt the power transfer signal to provide the required powerlevel. However, during the synchronization phase, the power control loopmay be used by the power receiver to adapt the signal level of the powertransfer time intervals to be different from that during the foreignobject detection time intervals.

For example, the power receiver may transmit power up requests until thesynchronizer 311 accurately detects signal level transitions (and e.g.until these transitions have a given magnitude). The power receiver maythen continue to transmit power up and power down requests to maintainthe level during the power transfer time intervals at this preferredlevel.

During the synchronization phase, the power control loop may accordinglybe used by the power receiver to generate suitable conditions for thesynchronization. A particular advantage of such an approach is that itmay use functionality already used for the actual power transferoperation. For example, the power transmitter may simply enter the powertransfer phase setting a predetermined signal level for the powertransfer signal during the foreign object detection time intervals andoperating the power control loop for setting the level during the powertransfer time intervals. This approach may be continued throughout thepower transfer phase regardless of whether the power receiver is in thesynchronization phase or not, and thus whether the power receiver isoperating in the first or the second mode. Indeed, the power transmittermay not be aware that the power receiver performs a synchronizationoperation or that this is even possible. Thus, the approach may providea simpler power transmitter operation and improved backwardscompatibility as no specific changes are required at the powertransmitter to support the synchronization phase.

In many embodiments, the system is arranged to determine parameters forthe repeating time frame by a negotiation/communication between thepower receiver and the power transmitter. This may be performed prior tothe start of the power transfer phase, and thus prior to the powertransfer phase, a set of parameters to use during the power transferphase may be determined.

The set of parameters (which in some cases may include only a singleparameter) determined by the communication prior to entering the powertransfer phase may include one or more of a duration of the foreignobject time intervals; an interval between foreign object timeintervals; and a signal level for the foreign object detectionintervals.

Thus, the power transmitter and the power receiver may employ apreparation phase or time interval prior to the power transfer phase inwhich they communicate with each other in order to determine one or moreparameters for the subsequent power transfer phase operation. Theoperation in the power transfer phase may then be based on theparameters determined during this preparation phase.

For example, in many embodiments, the power receiver may transmit one ormore messages to the power transmitter requesting a parameter value tobe applied during the power transfer phase. For example, it may requesta specific signal level of the generate electromagnetic signal or aspecific timing property of the foreign object detection time intervals.The power transmitter and power receiver may then proceed to apply theseparameter values and specifically these timing settings for theconsequent operation.

In many embodiments, the power transmitter and power receiver may inparticular be arranged to communicate in order to establish a durationof the foreign object detection time intervals and/or the durationbetween foreign object detection time intervals, and typically theduration between consecutive foreign object detection time intervals.

The approach may provide improved trade-offs and may in particular allowthe operation to be adapted to the specific properties of the individualdevices, and in many embodiments to the specific characteristics of theindividual power transfer operation. For example, the timing may beadapted to reflect the power level of the power transfer.

The approach may address that the duration of the foreign objectdetection time interval may affect the behavior of not only the powertransmitter and the power receiver but also potentially of the end load,such as e.g. a device that is powered from the power receiver.

Often, if the foreign object detection time interval is too long, thedevice may suffer from a decreased effective supply voltage. This isespecially relevant when the energy storage of the device is limited,such as for example where a capacitor at the input of the device (outputof the power receiver) is smaller than desired. The device must be ableto bridge the time during which the power transfer is interrupted, andthis typically requires a capacitor that is relatively large (it will benoted that in many embodiments, the power receiver may itself comprisesuch a capacitor as well as potentially voltage regulation to provide aconstant output voltage. However, this just means that the describedissue will be relevant for the power receiver rather than the externalload device).

If the time slot is too short, the power transmitter may not be able toperform the specific operation acceptably. For example, the powertransmitter may not be able to perform a FOD-measurement with sufficientaccuracy, e.g. because the measurement signal has not been stabilizedwhen the measurements are executed, or because an insufficient numbersamples can be taken. As another example, a foreign object detectiontime interval which is too short may not provide a sufficientcommunication bandwidth, e.g. it may not be possible to communicateenough data to support power control reporting and the provision ofother measurement results.

The optimal duration of the foreign object detection time interval mayaccordingly depend on a number of characteristics and properties, suchas the specific operating parameters and implementation of the powerreceiver. In some embodiments, the power receiver may accordinglytransmit a message to the power transmitter and the power transmittermay be arranged to adapt the timing of the foreign object detection timeinterval in response to this message.

The message may specifically explicitly be a request for a givenduration of the foreign object detection time interval. In manyembodiments, the power receiver may evaluate the operating conditions,such as the power being drawn by the external load and may calculate amaximum time during which the energy reservoir/capacitor is able tomaintain sufficient charge to prevent the supply voltage to the load todrop too much. For example, the maximum duration may be twice as highfor a load of 1 A compared to a load of 2 A. The power receiver may thustransmit a request for a duration which is twice as high for 2 A than itis for 1 A load.

As another example, suitable values for the duration of the foreignobject detection time interval may be predetermined for the powerreceiver, for example during the manufacturing phase. For example, thepower receiver may be a battery charger with a maximum charge current.The corresponding time duration for which the built in capacitor canretain sufficient charge to provide the maximum charge current can bedetermined during the design phase and stored permanently in the powerreceiver during the manufacturing phase. When initiating power transferwith a power transmitter, the power receiver can retrieve this value andtransmit a request for the foreign object detection time intervalduration to the power transmitter. The power transfer phase can thenproceed using a repeating time frame with foreign object detection timeintervals in accordance with the stored value. As power receivers mayvary very substantially in the requirements and functions, this mayallow the power transmitter and the power transfer operation to adapt tothe individual characteristics of the power receiver.

In some embodiments, the system may be arranged to set a durationbetween foreign object detection time intervals based on a messagetransmitted from the power receiver to the power transmitter. The systemmay specifically set the duration between foreign object detection timeintervals of consecutive repeating time frames and may effectively adaptthe duration of the foreign object detection time interval in responseto messaging from the power receiver to the power transmitter.

In order to provide sufficient average power transfer, the peak powerlevel of the power transfer during the power transfer time intervalsincreases the shorter the duration of these are. In many embodiments,the power transfer level may be limited (by the power transmitter orpossibly by the power receiver which may only be designed to extract agiven maximum amount of power). In such cases, the power receiver maytransmit a request for a duration between foreign object detection timeintervals which is sufficient to ensure that the capacitor will be fullycharged before the onset of the next repeating time interval (this isparticularly appropriate for embodiments in which the power transmitterswitches off the power transfer signal during the foreign objectdetection time intervals).

In some embodiments, a single request may be transmitted relating toboth the duration of the foreign object detection time interval and theduration between these. For example, in some embodiments, the repeatingtime frame may have a constant duration and the power receiver mayrequest a specific duty cycle to be applied.

In many embodiments, the power transmitter is arranged to impose aminimum duration requirement on the duration of the foreign objectdetection time interval. This minimum duration may be used to ensurethat the operation that is to be performed in the foreign objectdetection time interval actually has sufficient time to achieve thedesired result. For example, it may ensure that the foreign objectdetection can be performed with sufficient reliability (includingsufficient time for setting up and stabilizing the measurement signal).As another example, the power transmitter may be arranged to require aminimum duration in order for the communication to have sufficientbandwidth.

In many embodiments, the power transmitter is arranged to impose amaximum duration requirement on the duration between foreign objectdetection time intervals. This maximum duration may be used to ensurethat the operation that is to be performed in the foreign objectdetection time interval is performed sufficiently frequently. Forexample, it may ensure that the foreign object detection is performedwith sufficiently high frequency to ensure that the emergence of aforeign object will be detected before this can be heated tounacceptable levels. As another example, it may ensure thatcommunication is performed sufficiently frequently (e.g. enabling asufficient update rate for the power control loop).

As yet another example, in some embodiments, the measurements forforeign object detection may be spread over multiple reduced powerintervals to improve the accuracy and/or to add some redundancy. Thisenables more precision for foreign object detection. If the duration ofa reduced power time is short, and thus only allows for a small numberof samples/measurements, the duration between foreign object detectiontime intervals may be short in order to compensate and enable asufficient amount of samples/measurements to be acquired within a givenrequired time that ensures that a foreign object is detected before itheats up too much.

Similarly, the power receiver may be arranged to impose restrictions onthe timing values. For example, the power receiver may determine adesired value for the duration of the foreign object detection timeinterval subject to a maximum value that ensures that sufficient powercan be provided to an external load without the discharging of theenergy reservoir (typically a capacitor) resulting in unacceptablevoltage drops.

Similarly, the power receiver may, as mentioned above, determine adesired value for the duration between foreign object detection timeintervals subject to a minimum value that ensures that the powerreceiver capacitor can be fully recharged.

In many embodiments, the timing properties of the repeating timeinterval will be subject to requirements imposed by both the powerreceiver and the power transmitter. Typically, both the powertransmitter and the power receiver will have requirements that mustsimultaneously be met in order for the timing value to be adopted. Forexample, the setting of the duration of the repeating time intervaland/or the duration between consecutive repeating time intervals issubject to the values meeting requirements of both the power transmitterand the power receiver.

Further, in many embodiments, this may typically be one of the devices(i.e. the power receiver or the power transmitter) imposing arestriction on the maximum value, and the other device imposing arestriction on the minimum value of the timing property being set.

Specifically, as explained previously, in many embodiments, the durationof the foreign object detection time interval may be subject to aminimum duration imposed by the power transmitter and a maximum durationimposed by the power receiver.

Similarly, in many embodiments, the duration between foreign objectdetection time intervals may be subject to a maximum duration imposed bythe power transmitter and a minimum duration imposed by the powerreceiver.

Such implementations may impose efficient control of the suitabletimings for the foreign object detection time intervals in manyembodiments and may allow for reduced complexity and easier interworkingwith both devices independently ensuring that the foreign objectdetection time intervals will have timing properties that allowacceptable performance for both devices, and thus for the overall powertransfer.

The exact approach and message exchange used to set the timingproperties of the foreign object detection time intervals depends on thepreferences and requirements of the individual embodiment, and differentapproaches may be used in different systems.

However, in many systems, such as typically for Qi type implementations,the approach is based on the power receiver transmitting requests forsuitable timing values and the power transmitter accepting or rejectingthe rejected values.

Similarly, the power receiver may request a given signal level to beapplied during the foreign object detection time intervals by sending anindication during the preparation phase. This request may be based onmeasurements indicating the impact that the friendly metal of the powerreceiver will have on the generated signal, on any minimum powerrequired to be provided to the power receiver during the foreign objectdetection time intervals, to prevent an overvoltage, etc. The requestwill typically be based on estimations of a nominal power transmitterand operating conditions and therefore will tend to reflect a worst casescenario. However, it may in many embodiments be used to provide aninitial setting which can then be refined during the power transferphase in response to actual measurements of the operating conditions.

An advantage of the approach is that the impact of the friendly metal ofthe power receiver on a predetermined electromagnetic field can bedetermined e.g. during the manufacturing or design phase. The powerreceiver can then report this value to the power transmitter which maycompensate for the effect of the power receiver when performing theforeign object detection. This compensation may be very accurate if thepower receiver is subjected to a corresponding electromagnetic fieldstrength during the foreign object detection test.

FIG. 7 illustrates a (simplified) example of how the system may operate.

Initially, prior to the power transfer phase, a preparation time slot,PREP TS, is implemented. During this preparation time slot, the powerreceiver disconnects the load 303.

This means that no (or a very little) power is delivered to the load andthus extracted from the signal generated by the power transmitter. Thepower transmitter may control the drive signal to the transmitter coil103 to establish a situation at which following conditions preferablyapply:

-   -   The influence of the friendly metal of the power receiver device        on the magnetic field is known by the power receiver or can be        accurately determined by the power receiver. E.g. the power        dissipation in the friendly metal for a given amplitude and        frequency of the magnetic field is known by the power receiver.    -   The power transmitter can accurately measure the combined        influence of the friendly metal and a foreign object if present.        E.g. the transmitted power can be accurately determined by the        power transmitter.

As a result, the system can accurately determine the influence of aforeign object on the magnetic field. This influence preferably isrelated to the expected increase of temperature caused by the powerdissipation in the foreign object caused by its exposure to the magneticfield of the transmitter coil 103 when the power transmitter isproviding power to the power receiver.

To establish the above situation, the power receiver can provideinformation on an appropriate magnetic field, e.g. by communicating itstype, the allowed frequency range and optionally the required amplitudeof the AC signal for the transmitter coil 103. The latter of coursedepends on the design of the transmitter coil 103. In addition, thepower receiver could have a measurement coil of which the inducedvoltage gives a good indication of the field to which the friendly metalof the device is exposed. In that case the power receiver could providecontrol information to the power transmitter to control the field to alevel at which the influence of the friendly metal can be accuratelydetermined by the power receiver.

Once the above situation has been established, the power transmitter canstore the setting of the drive signal, and the expected influence of thefriendly metal as determined by the power receiver.

In addition, communication may be performed to determine suitable timingparameters for the repeating time frame, such as the duration of therepeating time frame and of the foreign object detection time intervals.

At the end of the preparation phase, a set of timing parameters and aforeign object detection signal level may have been set. Specifically,the preparation phase may have determined measurement signal conditionsfor the foreign object detection time intervals, the duration of theforeign object detection time intervals, and the time between theforeign object detection time intervals.

The power transmitter will then use these values during the powertransfer phase. However, some variation/uncertainty will typically bepresent, e.g. with respect to the exact timing of transitions betweenthe power transfer time interval and the foreign object detection timeintervals.

The system may then enter the power transfer phase in which therepeating time frame is applied. In the example of FIG. 7, the repeatingtime frame starts with a foreign object detection time interval followedby a power transfer time interval. The system starts in thesynchronization phase in which the load 303 is disconnected during theentire repeating time frame including both the foreign object detectiontime interval and the power transfer time interval. This is in FIG. 7indicated by the term NP (No Power).

The system further performs synchronization and when the reliabilitymeasure indicates that this has been achieved to a desiredreliability/level while keeping the power signal at a level at which noovervoltage occurs at the power receiver, the system switches to thesecond mode of operation corresponding to normal power transferoperation. In this mode, the load 303 is still disconnected during theforeign object detection time intervals but is connected during thepower transfer time intervals. This is indicated by the term “PWR”(Power) in FIG. 7. For simplicity and brevity, FIG. 7 illustrates thesynchronization phase to only comprise a single repeating time frame butit will be appreciated that it typically includes plurality of repeatingtime frames (in many applications the synchronization phase may includeno less than 10 or 20 repeating time frames (depending on whensynchronization is deemed to have been reached)).

As an example of the specific operation during the foreign objectdetection time intervals includes the power receiver having the load 303disconnected as previously discussed. This means that no (or verylittle) power is delivered to the load/power receiver. The powertransmitter applies the stored setting of the drive signal and measuresthe combined influence of the friendly metal and any foreign objectpresent. It compares this influence with the stored expected influenceof the friendly metal to determine the impact of the foreign object. Itmay then determine the maximum amplitude of the drive signal/powertransfer signal in relation to the frequency at which it regards thesituation to be safe, meaning at which it expects the temperature riseof a foreign object to be within safe limits. The power transmitter willthen limit the power transfer signal to this maximum level and report awarning if the power receiver tries to control the power transfer signalto a level above this maximum. Thus, in this example, the power transferphase is not terminated in the presence of a foreign object but ratherthe maximum power transfer signal level is reduced to a level ensuringthat the temperature increase in the foreign object is restricted toacceptable levels.

If the power transmitter detects a change in the combined impact of thefriendly metal and the foreign object, it may return to the preparationphase to re-establish the conditions for the foreign object detectionmeasurement. To prevent early triggers for returning to the preparationtime-slot, the power transmitter can combine the results of multiple FODtime slots (e.g. apply an averaging window) and/or adjust the drivesignal within certain margins.

In many embodiments, the synchronizer 311 is arranged to perform thesynchronization based on the set of parameters determined during thepreparation phase. For example, the determined parameters may be used asstarting parameters for the synchronization and thus the initial valuesof the timing parameters may be set to correspond to those determined inthe preparation phase. For example, the first estimate of the timetransitions from the foreign object detection time interval to the powertransfer time interval (and vice versa) may be determined from theagreed duration of the foreign object detection time intervals and theduration between these (the duration of a repeating time frame). Theseparameters may then be used as the initial parameters for thesynchronization, e.g. they may be used as the initial parameters for aphase locked timing loop with the initial error values then beingdetermined to reflect the difference between the measured leveltransition times and the predetermined values.

In other embodiments, the values determined during the preparation phasemay impose restrictions on the synchronization. For example, thesynchronizer 311 may fixedly set the duration of the foreign objectdetection time intervals and the duration between the foreign objectdetection time intervals to the values determined during the preparationphase. The synchronizer 311 may then adapt the timing offset, i.e. thetime instant for the transitions, to achieve the best results (smallesterror) under these conditions.

It will be appreciated that different approaches may be used fordetermining the reliability measure. In many embodiments, thereliability measure may be determined based on an error signalindicating the difference between the current values and the measuredvalues. For example, the (averaged/low pass filtered) error signal of aphase locked timing loop may provide a good indication of the accuracyand reliability of the synchronization. In other embodiments, thereliability measure may alternatively or additionally be determined independence on the variance of the timing values. For example, thesynchronization may initially be relatively unreliable and therefor thechanges in e.g. the timing of the beginning of a foreign objectdetection time interval may vary substantially. However, as thesynchronization becomes more accurate, the variation may settle on thecorrect value and the changes and variations may be reduced.

In some embodiments, the determination of the reliability measure maytake into account how long the power receiver has operated in the firstmode. In some embodiments, it may be considered that the synchronizationis initially reliable but that it will improve with time. In someembodiments, the reliability measure may be generated to be indicativeof a lower reliability for shorter durations than for longer durations.As a low complexity example, the reliability measure may be set toindicate unreliable synchronization until the synchronization operationhas been active for a given time. This time will typically depend on thedynamic properties of the synchronization, such as on the dynamic timingloop properties (e.g. the adaption rate). After this time, thereliability measure may be set to a value that depends on the size ofthe error signal. Such an approach may prevent a reliability measureindicating a high reliability due to a coincidental initial low errorsignal before the loop has properly adapted to the timing to result in aswitch to the second mode of operation. This may prevent the powerreceiver shifting to the second mode before synchronization has properlybeen achieved.

In some embodiments, the reliability measure may be determined inresponse to the parameters determined during the preparation phase. Forexample, the synchronizer 311 may be arranged to freely synchronize tothe repeating time frame based on e.g. detections of level transitions.The resulting timing properties, specifically the resulting duration ofthe foreign object detection time intervals and the interval betweenthese, may then be compared to the values determined during thepreparation phase. The reliability measure may be generated to reflecthow closely the synchronization values match the values of thepredetermined phase (which in this case may be considered to correspondto the values known to be applied by the power transmitter).

In some embodiments, the synchronizer 311 may be arranged to determinethe reliability measure for the synchronization based on a comparison ofsignal levels for the power transfer signal during power transfer timeintervals and foreign object time intervals, respectively. Such anapproach may in particular be suitable for determining a reliabilitymeasure during the power transfer phase after any potentialsynchronization phase, during “normal” power transfer operation.

In many embodiments, the synchronizer 311 may continuously synchronizeto the repeating time frame of the power transfer signal during thepower transfer phase in order to track variations (either in the timingof the power transmitter or of the local time reference) throughout apotentially very long power transfer operation. As previously described,such a synchronization may be based on detecting power level transitionsat the transitions between the time intervals. However, the powerreceiver may dynamically adapt the power level of the power transfersignal to match the load conditions and accordingly it is possible thatthe required power transfer signal level during the power transfer timeintervals becomes close to the predetermined level used during the powertransfer time intervals. In such a case, the synchronizer 311 may not beable to accurately detect the transitions and may therefore fail thesynchronization. Thus, in some examples, the synchronizer 311 may setthe reliability measure to a low level indicating poor synchronizationif it is not able to detect sufficiently large signal level steps.

The previous examples focused on the switch from the first mode to thesecond mode when detecting that the reliability measure indicates thatthe synchronization is sufficiently reliable. However, alternatively oradditionally, the mode controller 313 may be arranged to switch thepower receiver from the second mode to the first mode based on thereliability measure. Specifically, if the reliability measure fallsbelow a threshold during the power transfer phase and when in the secondmode, thereby indicating that the synchronization is no longer reliable,the mode controller 313 may switch the power receiver back to the firstmode of operation. Thus, as a specific example, if the synchronizationreliability becomes low during the “normal” power transfer operation,the mode controller 313 may switch the power receiver into the firstmode where the power load is also disconnected during the power transfertime intervals thereby ensuring that damaging overvoltage conditionscannot occur. The power receiver may further initiate a dedicatedsynchronization process and e.g. communicate with the power transmitterto apply parameters suitable for such a synchronization.

In some embodiments, the power receiver may in the first mode ofoperation be arranged to take actions that are likely to improve thesynchronization operation. Indeed, in many embodiments, the powerreceiver may proceed to only disconnect the load 303 during the foreignobject detection time intervals and still keep it connected during thepower transfer time intervals, i.e. there may be no specific differencesin the load operation between the first and the second modes. However,the power receiver may be arranged to modify the synchronizationoperation and mays specifically communicate with the power transmitterto change parameters of the power transfer signal that will facilitatesynchronization by the power receiver.

As a specific example, the power receiver controller 301 may be arrangedto communicate with the power transmitter to cause this to change thesignal levels of the power transfer signal/generated electromagneticfield such that the difference between the power transfer time intervalsand the foreign object detection time intervals are increased therebyfacilitating detection of the transitions between the different types ofintervals.

For example, as previously described, the power receiver controller 301is arranged to implement a power control loop and to transmit powercontrol messages to the power transmitter. The power transmitter adaptsthe signal level during the power transfer time intervals in response tothese messages, and the power receiver controller 301 is arranged togenerate the requests to cause the extracted power to match thatrequired by the power receiver to feed the load 303.

However, if the power receiver is in the second mode (and normal powertransfer is ongoing) and the reliability measure falls below a thresholdindicating that the synchronization is no longer sufficiently accurate(e.g. because the levels in the foreign object detection time intervaland the power transfer time interval are almost equal), the modecontroller 313 may switch the power receiver to the first mode ofoperation in which it changes the operation for the power controlmessages such that they are generated to result in a large(r) differencebetween the foreign object detection time intervals and the powertransfer time intervals. Thus, even if the power level is sufficient (ortoo high) for the power receiver to power the load 303, the powerreceiver controller 301 may continue to transmit power up requests inorder to increase the difference between the signal levels for theforeign object detection time intervals and the power transfer timeintervals. For example, power up requests may be transmitted until thedetected difference is sufficiently high and the power control may thenbe continued to maintain this level. Thus, in such an example, the powercontrol operation may switch from being based on parameters of the powertransfer operation (the required power level) to be based on parametersof the synchronization (the signal level step between the timeintervals).

Such an approach may be particularly advantageous for maintainingreliable synchronization throughout the power transfer phase, and may beapplicable e.g. to embodiments where no initial synchronization phase isperformed when entering the power transfer phase. For example, the powertransfer phase may simply start based on initial parameter valuesdetermined during the preparation phase without requiring any initialfine tuning of the synchronization. The power transfer phase mayaccordingly be started directly with the power receiver in the secondmode of operation. However, if the synchronization during the powertransfer phase is detected to become inaccurate/unreliable, the modecontroller 313 may switch the power receiver to the first mode ofoperation to perform (re)synchronization.

Such an approach will typically only be applied when the connected loaddoes not extract a lot of power (has a relatively high resistive value).

Indeed, directly starting the power transfer phase in the second mode ofoperation is that for a high power demand, the power receiver seeks torapidly control the power transmitter to provide a high signal level,because otherwise the voltage at the power receiver may drop below alevel at which it can operate. The external load level is normally notunder the control of the power receiver; the receiver has to deal withthe given load.

The approach may provide particularly efficient operation in the examplewhere the synchronization during the power transfer phase is performedbased on detecting signal level variations between the foreign objectdetection time intervals and the power transfer time intervals.

As another example, in some embodiments, the power receiver controller301 may be arranged to limit a rate of change for power levels when inthe first mode of operation to a lower level than when in the secondmode of operation. For example, when the power receiver is operatingduring “normal” power transfer and in the second mode of operation, thepower receiver may transmit power control messages which allow a fastadaptation of the power control loop so that it can follow fastvariations. However, if the reliability measure indicates that thesynchronization is not reliable, e.g. due to a determination that thesignal level changes between the power transfer time intervals andforeign object detection time intervals is low, the mode controller 313may switch the power receiver to the first mode of operation in whichthe rate of change may be substantially limited. Specifically, the powercontrol loop dynamics may be changed to implement a very slow loop andmay only gradually increase the power level in order to allow time for are-synchronization and to prevent an overvoltage condition at the powerreceiver when the load is disconnected that may otherwise have beencaused by a fast increase of the power signal while the synchronizationmay still be unreliable. In many embodiments, the adaptation may beasymmetric such that fast reduction in the power level is supported butonly very slow increases are possible.

In other embodiments, the power receiver controller 301 may in such asituation be arranged to restrict the power transmitter to provide amaximum power signal level when switching to the first mode, i.e. it mayallow a higher power signal level when in the second mode than when inthe first mode of operation.

In the system, the power receiver may when operating in the second modebe arranged to disconnect the load 303 shortly after the start of theforeign object detection time intervals and it may be arranged toreconnect it shortly before the end of the foreign object detection timeintervals with both the connection and the disconnection beingdetermined based on the local time reference as synchronized to therepeating time frame of the power transfer signal. For example, thestart and end times of the foreign object detection time intervals maybe determined and the switch times for disconnecting and reconnectingmay be set to these but typically with a short predetermined time offsetto ensure that the transitions always occur within the foreign objectdetection time intervals.

In some embodiments, the power receiver may comprise a currentrestrictor arranged to restrict the current to the load whenreconnecting this. Specifically, the current restrictor may be arrangedto limit the rate of change for the current to a given limit such thatonly a gradual increase is achieved instead of risking a large inrushcurrent when reconnecting the load 303 (which e.g. may occur if the load303 includes a large capacitive component). The current restrictor maye.g. be implemented in the form of an inductor.

FIGS. 8-10 may be used to illustrate circuit examples for such anapproach with the load 303 having both a resistive load component Rloadand a significant capacitive load Cload.

In the examples, the load 303 is disconnected at the start of theforeign object detection time interval corresponding to the switch Sopening. In the example, the voltage over Cbridge stays constant duringthe foreign object detection time interval. However, the voltage overCload reduces due to Rload (effectively Cload may act as an energyreservoir powering the load represented by Rload during the time theload 303 is disconnected, i.e. during the foreign object detection timeinterval).

In this situation, if no current restriction is included as in theexample of FIG. 8, a relatively high inrush current may flow fromCbridge to Cload to re-balance these voltages.

A solution addressing such an issue is to restrict the current change.e.g. by including an inductor between the capacitors Cbridge and Cloadto avoid such surges. Another, and often preferred approach may be toimplement a buck converter using S as the switch element of the buckconverter. Such an example is given in FIG. 9 where Dbuck and Lbuck areintroduced to create the buck converter. In many embodiments, this maynot substantially increase complexity as such components are oftenalready in place since most applications require some post control ofthe voltage across the load Rload anyhow.

It will be appreciated that the above description for clarity hasdescribed embodiments of the invention with reference to differentfunctional circuits, units and processors. However, it will be apparentthat any suitable distribution of functionality between differentfunctional circuits, units or processors may be used without detractingfrom the invention. For example, functionality illustrated to beperformed by separate processors or controllers may be performed by thesame processor or controllers. Hence, references to specific functionalunits or circuits are only to be seen as references to suitable meansfor providing the described functionality rather than indicative of astrict logical or physical structure or organization.

The invention can be implemented in any suitable form includinghardware, software, firmware or any combination of these. The inventionmay optionally be implemented at least partly as computer softwarerunning on one or more data processors and/or digital signal processors.The elements and components of an embodiment of the invention may bephysically, functionally and logically implemented in any suitable way.Indeed the functionality may be implemented in a single unit, in aplurality of units or as part of other functional units. As such, theinvention may be implemented in a single unit or may be physically andfunctionally distributed between different units, circuits andprocessors.

Although the present invention has been described in connection withsome embodiments, it is not intended to be limited to the specific formset forth herein. Rather, the scope of the present invention is limitedonly by the accompanying claims. Additionally, although a feature mayappear to be described in connection with particular embodiments, oneskilled in the art would recognize that various features of thedescribed embodiments may be combined in accordance with the invention.In the claims, the term comprising does not exclude the presence ofother elements or steps.

It will be appreciated that the reference to a preferred value does notimply any limitation beyond it being the value determined in the foreignobject detection initialization mode, i.e. it is preferred by virtue ofit being determined in the adaptation process. The references to apreferred value could be substituted for references to e.g. a firstvalue.

Furthermore, although individually listed, a plurality of means,elements, circuits or method steps may be implemented by e.g. a singlecircuit, unit or processor. Additionally, although individual featuresmay be included in different claims, these may possibly beadvantageously combined, and the inclusion in different claims does notimply that a combination of features is not feasible and/oradvantageous. Also the inclusion of a feature in one category of claimsdoes not imply a limitation to this category but rather indicates thatthe feature is equally applicable to other claim categories asappropriate. Furthermore, the order of features in the claims do notimply any specific order in which the features must be worked and inparticular the order of individual steps in a method claim does notimply that the steps must be performed in this order. Rather, the stepsmay be performed in any suitable order. In addition, singular referencesdo not exclude a plurality. Thus references to “a”, “an”, “first”,“second” etc. do not preclude a plurality. Reference signs in the claimsare provided merely as a clarifying example shall not be construed aslimiting the scope of the claims in any way.

1. A power receiver comprising: a synchronizer circuit, wherein thesynchronizer circuit is arranged to perform a synchronization bysynchronizing a time reference to a repeating time frame, wherein therepeating time frame comprises at least one power transfer interval andat least one detection interval; a load controller circuit, wherein theload controller circuit is arranged to disconnect a load, wherein theload is disconnected during at least a portion of the at least onedetection interval, wherein a timing of the disconnecting is dependenton the time reference; and a mode controller circuit, wherein the modecontroller circuit is arranged to switch between a first operationalmode and a second operational mode during a power transfer interval,wherein the switch is in response to a reliability measure of thesynchronization, wherein first power transfer parameters are used in thefirst operational mode and the second power transfer parameters are usedin the second operational mode, wherein the first power transferparameters are different from the second power transfer parameters. 2.The power receiver of claim 1, wherein the synchronizer circuit isarranged to perform a synchronization of the time reference to therepeating time frame wherein the mode controller circuit is arranged tocontrol the power receiver to operate in the first operational modewherein the mode controller circuit is arranged to switch the powerreceiver to the second operational mode in response to a the reliabilitymeasure for the synchronization exceeding a threshold.
 3. The powerreceiver of claim 2, further comprising a signal level controllercircuit, wherein the signal level controller circuit is arranged totransmit signal level requests to a power transmitter, wherein thesignal level requests are requests to set the signal level of the powertransfer signal, wherein the signal level controller circuit is arrangedto control the signal level of the power transfer signal during the atleast one power transfer interval, wherein the signal level of the powertransfer level is different from a signal level of the power transfersignal during the at least one detection interval when in the firstoperational mode wherein the synchronizer circuit is arranged tosynchronize between the at least one power transfer interval and the atleast one detection interval in response to signal variations.
 4. Thepower receiver of claim 1, wherein the synchronizer circuit is arrangedto determine the reliability measure of the synchronization in responseto a duration of operating in the first operational mode.
 5. The powerreceiver claim 1, further comprising an initiator circuit, wherein theinitiator circuit is arranged to determine a set of parameters for theat least one detection interval by communicating with the powertransmitter prior to, wherein the set of parameters comprises at leastone of a duration of the at least one detection interval, an intervalbetween two of the detection intervals and a signal level for the atleast one detection interval.
 6. The power receiver of claim 5, whereinthe synchronizer circuit is arranged to perform the synchronizationbased on the set of parameters.
 7. The power receiver of claim 5,wherein the synchronizer circuit is arranged to determine thereliability measure in response to a comparison of a timing parameterfor the at least one detection interval determined from the timereference and corresponding timing parameter of the set of parameters.8. The power receiver of claim 1, wherein the power receiver is arrangedto control the power transmitter so as to limit a signal level of thepower transfer signal to a level which is lower when in the firstoperational mode than in the second operational mode.
 9. The powerreceiver of claim 1, wherein the load controller circuit is arranged todisconnect a load from the power transfer signal during the at least onepower transfer interval when in the first operational mode but not whenin the second operational mode.
 10. The power receiver of claim 1,further comprising a power level controller circuit, wherein the powerlevel controller circuit is arranged to transmit power level requests tothe power transmitter, wherein the power level requests are requests toset a power level of power transfer signal, wherein the power levelcontroller circuit is arranged to limit a rate of change for powerlevels when in the first operational mode to a lower level than when inthe second operational mode.
 11. The power receiver of claim 1, whereinthe synchronizer circuit is arranged to determine the reliabilitymeasure of for the synchronization in response to a comparison of signallevels of the power transfer signal during the at least one powertransfer interval and the at least one detection interval.
 12. The powerreceiver of claim 1, wherein the mode controller circuit is arranged toswitch the power receiver from the second operational mode to the firstoperational mode in response to detecting that the reliability measureof the synchronization is below a threshold.
 13. The power receiver ofclaim 1, wherein the load controller circuit is arranged to reconnectthe load during the detection interval. wherein the load controllercircuit is arranged to reconnect the load during at least a portion ofthe power transfer phase, wherein a time to reconnect depends on thetime reference.
 14. The power receiver of claim 13, further comprising acurrent restrictor circuit, wherein the current restrictor circuits isarranged to restrict a current to the load when reconnecting the load.15. A method of operating a power receiver comprising: receivingwireless power from a power transfer signal; synchronizing a timereference to a repeating time frame, wherein the repeating time framecomprises at least one power transfer interval and at least onedetection interval; disconnecting a load during at least a portion ofthe at least one detection interval, wherein a timing of thedisconnecting is dependent on the time reference; and switching betweena first operational mode and a second operational mode for the at leastone power transfer interval in response to a reliability measure of forthe synchronization, wherein first power transfer parameters are used inthe first operational mode and second power transfer parameters are usedin the second operational mode.
 16. The method of claim 15, wherein thesynchronizing synchronizes the time reference to the repeating timeframe, wherein the switching operates the power receiver in the firstoperational mode, wherein the switching operated the power receiver inthe second operational mode in response to a the reliability measure offor the synchronization exceeding a threshold.
 17. The method of claim16, further comprising: transmitting signal level requests to a powertransmitter, wherein the signal level requests are request to set thesignal level within the power transfer signal; and controlling thesignal level of the power transfer signal during the at least one powertransfer interval to differ from a signal level of the power transfersignal during the at least one detection interval when in the firstoperational mode, wherein the synchronizing synchronizes between the atleast one power transfer interval and the at least one interval inresponse to signal variations.
 18. The method of claim 15, wherein thesynchronizing determines the reliability measure of for thesynchronization in response to a duration of operating in the firstoperational mode.
 19. The method of claim 15, further comprisingdetermining a set of parameters for the at least one interval bycommunicating with the power transmitter prior to entering a powertransfer interval; wherein the set of parameters comprises at least oneof a duration of the at least one detection interval, an intervalbetween two of the detection intervals and a signal level for the atleast one detection interval.
 20. A computer program stored on anon-transitory medium, wherein the computer program when executed on aprocessor performs the method as claimed in claim
 15. 21. A powerreceiver comprising: a synchronizer circuit, wherein the synchronizercircuit is arranged to perform a synchronization by synchronizing a timereference to a repeating time frame, wherein the repeating time framecomprises at least one power transfer interval and at least onedetection interval; a load controller circuit, wherein a power transferphase comprises at least two repeating time frames, wherein the loadcontroller circuit is arranged to disconnect a load, wherein the load isdisconnected during at least a portion of the at least one detectioninterval, wherein a timing of the disconnecting is dependent on the timereference; and a mode controller circuit, wherein the mode controllercircuit is arranged to switch between a first operational mode and asecond operational mode during a power transfer interval, wherein theswitch is in response to a reliability measure of the synchronization,wherein first power transfer parameters are used in the firstoperational mode and the second power transfer parameters are used inthe second operational mode, wherein the first power transfer parametersare different from the second power transfer parameters.